UN ITED STATES . I NSTALL ATI ON INFOR MATION . R ESI DEN TIA L
I N S T A L L AT I O N P R O C E D U R E S F O R
T H E R M A X ™ O R S T Y R O F O A M ™
B R A N D R I G I D I N S U L AT I O N
Closed Crawl Space
The closed crawl space is gaining popularity
for its effectiveness in reducing energy loss
and minimizing the potential for moisture
problems. Dow offers the following guidelines
for constructing a closed crawl space with an
insulation system that includes THERMAX™ or
STYROFOAM™ brand rigid insulation on interior
crawl space walls, a continuous poly vapor barrier
to prevent evaporation of ground moisture into
the crawl space, and a means to condition the
crawl space to keep the relative humidity below
70 percent to mitigate mold and mildew growth.
(See Figure 1.)
A b o u t T H E R M A X ™
R i g i d I n s u l a t i o n
THERMAX™ rigid insulation boards have a glassfiber-reinforced polyisocyanurate foam core,
with special laminated aluminum facers that
enhance moisture and damage resistance. This
reinforcement is chemically modified to provide
improved fire performance. THERMAX rigid
insulation boards can be left exposed in crawl
spaces per National Evaluation Services report
NER-681. THERMAX rigid insulation boards are
also Factory Mutual approved as a “Wall-Ceiling
Construction, FM Approvals Standard FM 4880,
Metal-Faced – Class 1 Fire Rated to a max, 30
ft. high.” Consult the local building codes in your
area before starting construction.
A b o u t S T Y R O F O A M ™ B r a n d
R i g i d I n s u l a t i o n
STYROFOAM™ brand rigid insulation boards are
made from extruded polystyrene foam insulation
and have exceptional water resistance, longterm thermal performance and high compressive
strength. ASTM C578 Type X and ASTM
C578 Type IV STYROFOAM Brand Extruded
Polystyrene Foam Insulation boards, with a
maximum nominal thickness of 2 inches (51mm),
may be installed on surfaces (wall, ceiling) of a
crawl space with no coverings (no thermal or
ignition barrier) applied to the crawl space side of
the foam plastic insulation per International Code
Council Evaluation Service (ICC-ES) Evaluation
Report ESR-2142.
Figure 1
Please note that closed crawl spaces may include
components not shown, such as other ventilation
options, floor drains, radon mitigation, etc.
Before You Begin
Before installing THERMAX™ or STYROFOAM™ brand rigid insulation boards on the interior crawl space wall, remove anything
that will interfere with installation. Best results are achieved when waterproofing or damp-proofing is applied from footer to grade
on the exterior of the crawl space wall. Install foundation drainage per code requirement.
The crawl space may contain utilities including, but not limited to, mechanical equipment, electrical wiring, fans, plumbing, gas
or electric hot water heaters, and gas or electric furnaces. When THERMAX or STYROFOAM brand insulation boards are used as
stated above in the crawl space, the following conditions must be met:
1. Under-floor (crawl space) ventilation is provided in accordance with IBC Section 1203.3 or IRC Section R408.1, as applicable; or
unvented crawl spaces are permitted under the conditions prescribed in IRC Section 408.3.
2. Combustion air is provided in accordance with IMC (International Mechanical Code) Section 701 and 703,
or IRC Section M1701 and M1703.
Installation on the interior of the crawl space wall should comply with 2009 IRC Section R408, 2009 IECC Section 402.2.9, Dow’s
ESR 2142 report and all other applicable code requirements. Additional useful information on closed crawl space systems can be
found at www.crawlspaces.org. Consult the local building codes in your area before beginning construction.
Materials Checklist
To insulate closed crawl spaces,
you will need:
• THERMAX™ or STYROFOAM™
brand rigid insulation boards –
select from any of the following:
- STYROFOAM Brand
Square Edge (25psi)
Insulation
- STYROFOAM Brand
Scoreboard (25psi) Insulation
- STYROFOAM Brand
Tongue and Groove
(25psi) Insulation
- THERMAX Sheathing
(with foil facing)
- THERMAX White Finish
Insulation
• Utility knife or small handsaw
• Straight edge
• Measuring tape
• Pencil
• Safety glasses
• Water-based adhesive mastic
• 6-mil poly liner
• Duct tape
• WEATHERMATE™
Construction Tape (optional)
• Spray urethane foam such as
GREAT STUFF PRO™ Gaps
& Cracks Insulating Foam
Sealant (optional)
• FROTH-PAK™ Foam Insulation
kit (optional)
• 6" galvanized nails
• 1" nailing tins
Cutting The Boards
THERMAX™ or STYROFOAM™ brand
rigid insulation boards may be cut
with a straight edge and utility knife or
with a small handsaw. (See Figures
2a and 2b.)
Figure 2a
Figure 2b
Installation
1. Attach a Type 1 Vapor Retarder such
as a 6-mil poly liner to the interior
stem wall with water-based adhesive
mastic. The edges of the vapor
retarder should extend at least 6
inches up the stem wall. Extend the
wall poly liner onto the ground about
1 to 2 feet from the wall. (See
Figure 3.)
Figure 3
2. Fasten the THERMAX™ or
STYROFOAM™ brand insulation
boards to the interior of the
crawl space wall using powerdriven masonry nails with 1-inch
washers or caps. The boards
shall be permanently fastened to
the interior of the stem wall and
extend downward from the top
of the stem wall to the exterior
finished grade level and then
vertically and/or horizontally
for at least 24 inches. In some
situations, an inspection strip may
be appropriate. An inspection strip
is a 2-inch to 3-inch strip around
the top of the stem wall without
insulation, which allows inspection
for termites. (Check with your local
code officials and pest control
operators regarding this option.)
Space fasteners approximately
2 feet on center vertically and
4 feet on center horizontally.
(See Figure 4a.)
Note: If the distance between
the exterior grade level and the
crawl space floor is greater than
24 inches, the rigid insulation
board on the wall should extend at
least 24 inches below the exterior
grade. (See Figure 4b). If the
2
distance between the exterior grade
level and the crawl space floor is less
than 24 inches, the board on the wall
should extend to the crawl space floor
and then horizontally on the floor so
the total board length below exterior
grade (both vertically and horizontally)
equals at least 24 inches. (See
Figure 4c.)
3. Insulate the rim joist and sill plate.
There are three options for this step:
a. Option 1: Use FROTH-PAK™
Foam Insulation kits up to
2 inches in thickness. (See
Figure 5a.)
b. Option 2: Use STYROFOAM™
brand rigid insulation
boards up to 2 inches in
thickness, cut to fit between
the floor joists, and seal
the edges of the foam with
GREAT STUFF PRO™ Gaps &
Cracks. (See Figure 5b.)
c. Option 3: Use THERMAX™
rigid insulation boards up to
4 inches in thickness, cut to fit
between the floor joists, and
seal the edges of the foam
with GREAT STUFF PRO Gaps
& Cracks. (See Figure 5b.)
Figure 5a
4. Seal all penetrations through
the crawl space wall with spray
urethane foam such as GREAT
STUFF PRO Gaps & Cracks. (See
Figure 6.) Joints between boards of
insulation may also be taped with
WEATHERMATE™ Construction
Tape, if desired.
5. Install a temporary “construction”
poly liner as soon as the house is
“dried in” (i.e., tar paper is on the
roof). Cover the floor area with poly,
but leave seams unsealed. This will
temporarily prevent moisture from
the soil from evaporating into the
crawl space.
6. Before installing the permanent poly
ground cover, temporary installation
of a small dehumidifier will help dry
out the moisture that already exists
in the crawl space. Set the drain
tube to discharge outside of the
crawl space.
7. When the house is near
completion, remove the temporary
“construction” poly and discard.
Figure 4a
Figure 4b
Figure 5b
Figure 6
8. Roll out and install a permanent
Type I Vapor Retarder such as a
6-mil poly liner. The vapor retarder
sheets should overlap each other
by at least 6 inches. Carefully seal
the seams at the wall poly liner with
duct tape and mastic. Also seal
all joints and penetrations in the
field. (See Figure 7.) Secure each
joint with duct tape, then brush on
a layer of mastic. Mastic should
extend at least 1 inch beyond the
tape on each side.
9. Secure vapor retarder to the ground
with a 6-inch galvanized spike
through 1-inch diameter nailing tins.
For reinforcement and a tight seal,
apply duct tape and mastic over
spikes and tins.
The steps outlined here are intended to
make the crawl space clean, dry, wellinsulated, airtight and vapor tight. There
are other important components in a
closed crawl space system. Consult IRC
Section 408 or www.crawlspaces.org for
other components such as ventilation
options.
Figure 4c
Figure 7
3
A d d i t i o n a l
R e c o m m e n d a t i o n s
and polyol. Do not breathe vapor
or mist. Use only in well-ventilated
areas or with proper respiratory
• Builders may wish to install a
humidity sensor in the crawl space,
wired to a digital readout and alarm
upstairs, to help homeowners
monitor relative humidity.
• To minimize damage to the poly
liner, install carpet runners from the
access door to the water heater,
furnace and other appliances as a
path for service technicians.
• The homeowner should be
informed that a closed crawl space
should be treated as part of the
main house and must not be used
for storage of lawn equipment,
pesticides, fuel, etc.
protection. Supplied air or an
approved air-purifying respirator
equipped with an organic vapor
sorbent and a particle filter may
be required to maintain exposure
levels below ACGIH, OSHA, WEEL
or other applicable limits. For
situations where the atmospheric
levels may exceed the level for
which an air-purifying respirator is
effective, use a positive-pressure,
air-supplying respirator (air line
or self-contained breathing
apparatus).
• Isocyanate is irritating to the
eyes, skin and respiratory system,
and may cause sensitization by
inhalation or skin contact.
S A F E T Y A N D
C O N D I T I O N S O F U S E
FROTH-PAK™ Foam Insulation
• Read the instructions and Material
Safety Data Sheets carefully before
use.
• FROTH-PAK™ Foam will adhere
to most surfaces and skin. Do not
get foam on skin. Wear gloves,
goggles or safety glasses, and
protective clothing. Cured foam
must be mechanically removed or
allowed to wear off in time.
• The contents are under pressure.
• FROTH-PAK™ Polyurethane
Spray Foam contains isocyanate,
hydrofluorocarbon blowing agent
www.insulateyourhome.com
FOR TECHNICAL INFORMATION: 1-866-583-BLUE (2583)
FOR SALES INFORMATION: 1-800-232-2436
THE DOW CHEMICAL COMPANY
Dow Building Solutions • 200 Larkin • Midland, MI 48674
GREAT STUFF PRO™ Gaps &
Cracks Insulating Foam Sealant
• Read the label and Material Safety
Data Sheet carefully before use.
• GREAT STUFF PRO™ Gaps &
Cracks contains isocyanate and a
flammable blowing agent. Vapor
may travel to other rooms. Ensure
adequate ventilation. Shut off all
pilot lights and extinguish open
flames; eliminate all sources of
ignition before use. Do not smoke
or use lighters or matches while
dispensing foam.
• Do not breathe vapor or mist. Use
in well-ventilated areas or wear
proper respiratory protection.
Isocyanate is irritating to the
eyes, skin and respiratory system,
and may cause sensitization by
inhalation or skin contact.
• GREAT STUFF PRO™ Gaps &
Cracks is very sticky and will
adhere to most surfaces and
skin. Do not get foam on skin.
Wear gloves, and safety glasses
or goggles. Cured foam must be
mechanically removed or allowed
to wear off in time.
• The contents are under pressure.
The can may burst if left in areas
susceptible to high temperatures,
such as motor vehicles, or near
radiators, stoves or other sources
of heat. Do not place can in hot
water. Do not puncture, incinerate
or store at temperatures above
120°F (49°C).
STYROFOAM™ Brand Extruded Polystyrene Foam Insulation is combustible. Protect from high heat sources. A protective barrier or thermal barrier may be required as specified in the appropriate building
code.
THERMAX™ Polyisocyanurate Sheathing is combustible and shall only be used as specified by the local building code with respect to flame spread classification and to the use of a suitable barrier.
FROTH-PAK™ Polyurethane Spray Foam contains isocyanate, hydrofluorocarbon blowing agent and polyol. Read the instructions and Material Safety Data Sheets carefully before use. Wear protective clothing,
gloves, goggles or safety glasses, and proper respiratory protection. Supplied air or an approved air-purifying respirator equipped with an organic vapor sorbent and a particle filter may be required to maintain
exposure levels below ACGIH, OSHA, WEEL or other applicable limits. Provide adequate ventilation. Contents under pressure. Consult the instructions and Material Safety Data Sheet carefully before use.
GREAT STUFF PRO™ Insulating Foam Sealant contains isocyanate and a flammable blowing agent. Read the label and Material Safety Data Sheet carefully before use. Eliminate all sources of ignition before
use. Wear gloves, and safety glasses or goggles. Provide adequate ventilation or wear proper respiratory protection. Contents under pressure.
CAUTION: These products are combustible and shall only be used as specified by the local building code with respect to flame spread classification and to the use of a suitable thermal barrier. For more information call Dow at 1-866-583-BLUE (2583) or contact your local building inspector. In an emergency, call 1-989-636-4400.
Building and/or construction practices unrelated to building materials could greatly affect moisture and the potential for mold formation. No material supplier including Dow can give assurance that mold
will not develop in any specific system.
NOTICE: No freedom from any patent owned by Dow or others is to be inferred. Because use conditions and applicable laws may differ from one location to another and may change with time, Customer is
responsible for determining whether products and the information in this document are appropriate for Customer’s use and for ensuring that Customer’s workplace and disposal practices are in compliance
with applicable laws and other government enactments. Dow assumes no obligation or liability for the information in this document. NO EXPRESS WARRANTIES ARE GIVEN EXCEPT FOR ANY APPLICABLE
WRITTEN WARRANTIES SPECIFICALLY PROVIDED BY DOW. ALL IMPLIED WARRANTIES INCLUDING THOSE OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE EXPRESSLY
EXCLUDED.
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®™
Trademark of The Dow Chemical Company (“Dow”) or an affiliated company of Dow
Form No. 179-07259-1009XBBI
McKAY200501
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